Frame structures of vehicle bodies of automobiles are configured from plural frame members obtained by press molding sheet metal (sheet steel is taken as an example of metal stock sheet in the explanation that follows). Such frame components are very important components for securing crash safety in automobiles. Known examples of frame members include side sills, cross members, and front pillars.
Such frame members have hat shaped cross-sections configured from a top plate section, two vertical walls joined to the both sides of the top plate section, and two flange portions joined to the two respective vertical walls. Most of such frame members have a hat shaped cross-section over a portion or the entirety thereof. High strength is desired in such frame members in order to improve crash safety performance and to achieve weight reduction in vehicles.
FIG. 19 is a perspective view of an example of a frame member 0 that has a hat shaped cross-section and that has a straight shape along the longitudinal direction in plan view and side view. FIG. 20 is a explanatory diagram of a front pillar 0-1 that is an example of a frame member having a hat shaped cross-section. FIG. 20A is a perspective view and FIG. 20B is a plan view. FIG. 21 is a perspective view illustrating a component 1 having a hat shaped cross-section and having a shape curved in an L shape along the longitudinal direction in plan view. In the present specification, “plan view” refers to viewing the front pillar 0-1 from a direction orthogonal to the top plate section that is the widest planar portion on the member (the direction of the white arrow in FIG. 20A, and the direction orthogonal to the page in FIG. 20B).
The frame member 0 is a side sill or the like out of frame members having a hat shaped cross-section, and has a shape that is substantially straight in the longitudinal direction as illustrated in the example of FIG. 19. In contrast thereto, the front pillar 0-1 has a shape like that illustrated in FIG. 20A and FIG. 20B. Namely, the front pillar 0-1 has a hat shaped cross-section and includes a shape curved in an L shape along the longitudinal direction in plan view as illustrated in FIG. 21, at the side of a lower portion 0-2 of the front pillar 0-1.
Out of such components, the frame member 0 is manufactured mainly by bending due to having a substantially straight shape along the longitudinal direction. Due to the peripheral length of the cross-section of the frame member 0 not changing much along the length direction, cracks and creases are not readily generated during press processing even when formed from high strength sheet steel having low extensibility, and molding is simple.
For example, Patent Document 1 describes a method that forms a press component having a hat shaped cross-section by bending. The method described in Patent Document 1 manufactures press components that have a hat shaped cross-section and that have a substantially straight shape along the longitudinal direction.
FIG. 22 is a perspective view illustrating a press component 1 manufactured by bending that has a hat shaped cross-section and that has a shape curved in an L shape along the longitudinal direction.
When the component 1 illustrated in FIG. 21 that has a hat shaped cross-section and that is curved in an L shape along the longitudinal direction by bending using the method described by Patent Document 1, creases are generated in the flange portion (portion A) at the outside of a curved portion 1a as illustrated in FIG. 22. Therefore, the component 1 is generally molded by drawing in press processing. In drawing, steel stock sheet is molded using a die, a punch, and a blank holder in order to control the amount of inflow of metal stock sheet and to suppress generation of creases.
FIG. 23 is an explanatory diagram illustrating a component 2 to be molded that is curved in an L shape along the longitudinal direction. FIG. 23A is a perspective view, and FIG. 23B is a plan view. FIG. 24 is a plan view illustrating the shape of a steel stock sheet 3 when drawing, and a crease inhibiting region B in a steel stock sheet 3. FIG. 25A to FIG. 25D are cross-sections illustrating a mold structure and a molding procedure for drawing. FIG. 26 is a perspective view of a drawn panel 5 that has been molded by drawing.
For example, molding employs a die 41, a punch 42, and a blank holder 43 as illustrated in FIG. 25A to FIG. 25D, in order to mold a component 2 that is curved in an L shape along the longitudinal direction in side view, illustrated in FIG. 23, by drawing.
First, as illustrated in FIG. 25A, the steel stock sheet 3 illustrated in FIG. 24 is positioned between the punch 42 and the blank holder 43, and the die 41. Next, for example, as illustrated in FIG. 25B, the crease inhibiting region B of the periphery of the steel stock sheet 3 (the hatching shaded portion of FIG. 24) is held strongly by the blank holder 43 and the die 41. Next, as illustrated in FIG. 25C, the die 41 is moved relatively in the direction of the punch 42. Then finally, as illustrated in FIG. 25D, the drawn panel 5 illustrated in FIG. 26 is molded by processing the steel stock sheet 3 by pressing the steel stock sheet 3 against the punch 42 using the die 41.
When doing so, the crease inhibiting region B at the periphery of the steel stock sheet 3 is held strongly by the blank holder 43 and the die 41. Therefore, in the steel stock sheet 3 obtained by the molded procedure, the steel stock sheet 3 is stretched in a region at the inside of the crease inhibiting region in a state under tensile load. It is therefore possible to mold while suppressing generation of creases. The unwanted portions at the periphery of the molded drawn panel 5 are then cut away so as to manufacture the component 2 illustrated in FIG. 23A and FIG. 23B.